1. Field of the Invention
This invention relates to directional couplers. More particularly, the invention is concerned with a cost efficient directional coupler having a variable coupling factor.
2. Description of Related Art
Directional couplers are useful for sampling and or measuring RF energy. The directional characteristic of directional couplers allows separate measurement and or sampling of the forward and reflected components of RF energy traveling along, for example, a coaxial cable. The coupling factor is a measure of how much of the total RF energy present in a main cable is coupled to an auxiliary cable, the remainder continuing along the main cable. Variable coupling factor functionality allows the level of sampling and or measurement to be adjusted.
Mathematical models for the electrical interaction between coupled lines of unequal cross section and coupled coaxial lines in particular are well known to those skilled in the art. Also, factors influencing directivity in a directional coupler are known.
Common for usage in high power RF systems are directional couplers with loose coupling values (30–50 dB) between a main power carrying line of large size (1⅝″ EIA to 8 3/16″ or waveguide) and a small size coupled line feeding a monitor or feedback circuit (interconnected using, for example, type N or TNC connectors).
Couplers implemented with a variable rather than fixed coupling factor have some advantages over fixed coupling factor couplers. For example, they can serve as a flexible test instrument and be field set for specific applications. They are also useful in high power low VSWR systems where monitoring forward power requires a low coupling factor in order to protect the detector but also a higher coupling factor to detect a typically much lower reflected power. They are also useful in a production environment where a single assembly can be stocked and rapidly adjusted to a range of desired coupling factors.
The typical approach for loosely coupled mechanically adjustable directional couplers is to use an electrically short (less than one quarter wavelength) coupled line whose proximity to the main line can be varied. By moving the coupled line closer to the main line the coupling is increased and by moving it farther away the coupling is decreased. The directivity of the coupler is then optimized for specific coupling values by rotating the coupled lines orientation with respect to the mainline. Orientations of 30° to 60° are typical. This design approach requires a coupled line assembly with two mechanical degrees of freedom (proximity and rotation) with respect to the mainline. The cost of manufacture of such an assembly may be relatively expensive. The fact that the coupled line is electrically short means that the coupling value is not flat over a broad frequency range, generally falling off at 6 dB per octave.
Competition within the coupler industry has focused attention on reduction of coupler materials and manufacturing costs.
Therefore, it is an object of the invention to provide an apparatus that overcomes deficiencies in the prior art.